1. Field of the Invention
This invention relates generally to communication systems and, more particularly, to a subscriber, or transceiver, unit that can be used in the cellular system and that can also be used with the wide area dispatch system, or in a system that is a combination of these systems.
2. Discussion of the Related Art
In the field of commercially available, portable communication systems, such as can easily be transported in the motor vehicle, at the present time several systems are in use for permitting the transfer of information from one subscriber to a second subscriber. The first system is the cellular system in which a plurality of local transmit/receive installations communicate with a multiplicity of local subscriber units. In this system, to communicate with another participant in the system or to be coupled to an independent communication system, such as the telephone system, the subscriber communication signals are received by the local antenna, transferred to a central unit for processing and appropriate action taken to contact the desired receiving unit. For example, if the target receiving subscriber unit is in a vehicle in a local area serviced by a different transmit/receive installation, the central processing or control system will transfer the communication signals to a telecommunications system. The telecommunications system will return the communication signals to the central processing system and the central processing system will transfer the communication signals to the local transmit/receive installation receiving unit in which the target subscriber unit is located, and the local transmit/receive installation will transmit the communication signals to the target subscriber unit. This system permits full duplex operation and has procedures for handing off the subscriber units to a different transmit/receive installation as a vehicle moves to another local area. The federally assigned frequencies include 333 channels for transmission in the frequency range 870.030 MHz through 879.990 MHz and 333 additional channels for transmission in the frequency range of 880.020 MHz through 889.980 MHz with channel separation frequency of 30 KHz. The channels for reception are spaced 45 MHz below the corresponding transmission channel.
In a second system, generally referred to as the wide area or dispatch system, the communication is accomplished directly from subscriber unit to subscriber unit however, under control of a central transmitter. That is, when the transmitting unit desires to communicate with a perspective receiving unit or units, a designated channel is assigned to both the receiving unit and the transmitting unit by the control system associated with the transmit/receive installation, thereby avoiding potential conflicts in the use of the communication channel. The Federally assigned frequencies for the dispatch system include transmission channels in the frequency range 856.0125 MHz through 865.9875 MHz with a 25 KHz channel separation. The channels for reception are 45 MHz below the corresponding transmission channel. A single transmit/receive installation is used to service an entire area as compared to the cellular system in which a plurality of local transmit/receive installation service local regions. The transmit/receive installation receives the signals from an originating subscriber unit and retransmits the signals to the target subscriber unit.
Federal regulations limit the particular service to which each of the assigned channel's designated frequency band can be used. In particular, the cellular system is not permitted to use the dispatch mode i.e. the mode of operation wherein the communication is made from subscriber unit to subscriber unit or to their place of business via direct radio communication. Therefore, a substantial advantage can be found in combining the two systems in that a wider variety of services, customizing the needs of the individual customer to the available services can be offered by the operator of the system. However, a further problem in the use of a communication combining the two communication systems described above is that, not only are different frequencies ranges assigned by federal regulations to each type of service, but the frequency separation between transmit channels is 25 kilocycles for the trunked dispatch wide area system, and is 30 kilocycles for the cellular system. Because the architecture of the subscriber unit for each service is structured on the separation frequency, the two types of units are generally not compatible.
A need has therefore been felt for a communication system employing a transceiver unit that can utilize the channels assigned both to the wide area or dispatch mode communication system and to the channels assigned to the cellular mode communication system.